Project Summary Mononuclear nonheme iron (MNHI) enzymes that active dioxygen (O2) catalyze an impressive variety of oxidative transformations in aerobic organisms. The proposed research seeks to gain a deeper understanding of biological O2-activation by developing synthetic models of two types of MNHI enzymes that oxidize cysteine- derived biomolecules: thiol dioxygenases (TDOs) and sulfoxide (SO) synthases. Both classes of enzymes are relevant to human metabolism and health. TDOs, such as cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO), play a central role in regulating concentrations of thiol compounds in mammalian cells. CDO initiates the catabolism of exogenous L-cysteine, and diminished levels of CDO activity have been implicated in neurological disorders in humans. The SO synthases are involved in the formation of S-containing natural products with therapeutic properties. For both types of enzymes, questions persist regarding the nature of the catalytic mechanisms and role of the active-site pocket in controlling substrate selectivity and product identities. The research described in this application addresses these unanswered questions through the bio- inspired synthesis and characterization of iron-containing systems that replicate key elements of enzymatic structure and function. In addition to mimicking the ligands bound directly to the enzymatic Fe(II) centers (i.e., the first coordination sphere), the proposal offers strategies for the incorporation of second-sphere groups into the synthetic models. By examining the impact of structural variations and second-sphere moieties on the O2 reactivities of the synthetic TDO and SO synthase models, it will be possible to derive structure-function correlations that are applicable to the enzymes themselves. Intermediates obtained from reaction of the iron- based models with O2 (or related species, such as superoxide or nitric oxide) will be isolated and characterized with a wide array of spectroscopic and computational techniques. In addition to the proposed synthetic efforts, spectroscopic studies of enzyme samples will be pursued to probe the order of substrate binding and the electronic structures of catalytically-relevant species. Interpretation of the enzymatic data will be facilitated by comparison to data collected for the corresponding model complexes. The specific aims of the proposed research are to: (i) explore the catalytic mechanism of CDO through synthetic, spectroscopic, and computational studies of active-site models, (ii) develop structural and functional SO synthase models that replicate the first and second coordination spheres, and (iii) perform spectroscopic studies of enzyme samples to elucidate the geometric and electronic structures of catalytically-relevant species. The interdisciplinary nature of the proposed project will provide undergraduate and graduate students at Marquette University with valuable training experiences in cutting-edge synthetic, spectroscopic, and theoretical methods.